Liquid crystals have found use in a variety of electro-optical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays. Liquid crystal displays have a number of unique useful characteristics, including low voltage and low power of operation. In such displays, a thin layer of liquid crystal material is placed between glass plates and the optical properties of small domains in the layer is controlled by the application of electric fields with high spatial resolution. These devices are based upon the dielectric alignment effects in nematic, cholesteric and smectic phases of the liquid crystal compound in which, by virtue of dielectric anisotropy, the average molecular long axis of the compound takes up a preferred orientation in an applied electric field. However, since the coupling to an applied electric field by this mechanism is rather weak, the electro-optical response time of liquid crystal based displays may be too slow for many potential applications such as in flat-panel displays for use in video terminals, oscilloscopes, radar and television screens. Fast optical response times become increasingly important for applications to larger area display devices. Insufficient nonlinearity of liquid crystal based displays can also impose limitations for many potential applications.
Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of ferroelectric liquid crystals (FLCs) of N. A. Clark and S. T. Lagerwall (1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures prepared using FLC materials having not only high speed response (about 1,000 times faster than currently used twisted nematic devices), but which also exhibit bistable, threshold sensitive switching. Such properties make FLC based devices excellent candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays.
Smectic C liquid crystal phases composed of chiral, nonracemic molecules possess a spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Myer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions. In ferroelectric liquid crystal display devices, like those of Clark and Lagerwall, appropriate application of an external electric field results in alignment of the chiral molecules in the ferroelectric liquid crystal phase with the applied field. When the sign of the applied field is reversed, realignment or switching of the FLC molecules occurs. This switching can be employed for light modulation. Within a large range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (P), and directly proportional to orientational viscosity. Fast switching speeds are then associated with FLC phases which possess high polarization density and low orientational viscosity.
A basic requirement for application of ferroelectric liquid crystals in such devices is the availability of chemically stable liquid crystal materials which exhibit ferroelectric phases (chiral smectic C*) over a substantial temperature range about room temperature. In some cases, the ferroelectric liquid crystal compound itself will possess an enantiotropic or monotropic ferroelectric (chiral smectic C*) liquid crystal phase. Ferroelectric liquid crystal mixtures possessing chiral smectic C* phases with useful temperature ranges can also be obtained by admixture of chiral, nonracemic compounds, designated ferroelectric liquid crystal dopants, into a liquid crystal shot material (which may or may not be composed of chiral molecules). Addition of the dopant can affect the ferroelectric polarization density and/or the viscosity of the C* phase and thereby affect the switching speed. Desirable FLC diopants are molecules which impart high ferroelectric polarization density to an FLC material without significantly increasing the orientational viscosity of the mixture.
Thermotropic liquid crystal molecules typically possess structures which combine a rigid core coupled with two relatively "floppy" tails (see Demus et al. (1974) Flussige Kristalle In Tabellen, VEB Deutscher Verlag fur Grundstoffindustrie, Lebzig for a compilation of the molecular structures of LC molecules). FLC materials have been prepared by the introduction of a stereocenter into one of the tails, thus introducing chirality. The first FLC compound to be characterized was DOBAMBC (Meyer et al., supra) which contains an (S)-2-methylbutyloxy chiral tail. Pure DOBAMBC exhibits a smectic C* phase with a ferroelectric polarization of -3 nC/cm.sup.2.
The structures and polarization of several know smectic C* materials, including several containing phenylbenzoate cores, have been summarized in Walba et al. 1986a) J. Amer. Chem. Soc. 108:5210-5221, which also discusses a number of empirical correlations between molecular structure and FLC properties. For example, this reference (and U.S. Pat. No. 4,556,727) reports FLC compounds which contain nonracemic 2-alkoxy-1-propoxy tail units, derived from lactic acid, coupled to 4-substituted phenylbenzoate cores: ##STR2## where R is a lower alkyl group containing one to three carbon atoms and R' is an alkyl group containing nine to twelve carbon atoms. These compounds possess monotropic smectic C* phases which display fast switching speeds at room temperature. It is also reported therein that certain eutectic mixtures containing these FLC compounds possess thermodynamically stable or enantiotropic smectic C* phases with high polarization density and fast electrooptical switching speeds.
Walba et al. (1986) J. Amer. Chem. Soc. 108:7424-7425 and Walba and Vohra, U.S. Pat. No. 4,648,073 disclose ferroelectric smectic liquid crystal compounds possessing a high ferroelectric polarization density having chiral tail units derived from (2,3)-alkyloxiranemethanols and achiral phenylbenzoate and biphenyl core units. The ferroelectric crystal materials reported have the following general formulas: ##STR3## R is an alkyl of one to seven carbon atoms and R' is an alkyl of five to twelve carbon atoms and Ar is phenylbenzoate or biphenyl.
Eidman and Walba, U.S. patent application Ser. No. 800,851, filed July 1, 1986, discloses chirally asymmetric liquid crystals possessing the phenylbenzoate core unit and 1-cyanoalkoxy chiral tails.
Walba and Razavi, U.S. patent application Ser. No. 911,096, filed Sept. 24, 1986 discloses chirally asymmetric compounds possessing a reverse ester phenylbenzoate core unit with 1-fluoro or 1-chloroalkyl group chiral tail units having the general formula: ##STR4## wherein R is an alkyl group of three to twelve carbon atoms, R' is an alkyl of five to twelve carbon atoms, and X is a chlorine atom or a fluorine atom. These materials impart the property of high polarization density in mixtures which display an FLC phase and are useful as FLC dopants.
Walba and Razavi, U.S. patent application Ser. No. 099,074, filed Sept. 21, 1987, discloses chirally asymmetric phenyl and biphenylbenzoates having chiral 2,3-epoxy alkyl or I-halo-2,3-epoxy alkyl tails which are useful as components of FLC materials. The compounds disclosed have the formula: ##STR5## where R' is an alkyl or alkoxyl group having three to fifteen carbon atoms, R is an alkyl group having three to fifteen carbon atoms, n=1 or 2, and Y is a halogen or hydrogen. It is also disclosed, therein, that 1-haloepoxides of formula A can impart higher polarization densities and higher switching speeds in FLC mixtures than their diastereomers of formula B. It is suggested that the difference in properties of A and B is due to the ##STR6## relative alignment of the epoxide and halogen bond dipoles in the isomers.
Higuchi et al. U.S. Pat. No. 4,695,651 disclose biphenyl-based diester liquid crystal compounds having the general formula: ##STR7## wherein R.sup.1 and R.sup.2 represent C.sub.1-18 alkyl, alkyl halide or aralkyl halide groups and X is --COOCH.sub.2 -- or --OCO--. Compounds in Which the R.sup.2 group contains an asymmetric carbon are disclosed, although no specific stereochemistry is specified. The liquid crystal materials disclosed are said to display ferroelectricity. In related work, Higuchi et al. U.S. Pat. No. 4,592,858 disclose chiral smectic liquid crystal compounds having biphenyl cores attached by an ester linkage to an optically active group as in the formula: ##STR8## wherein the carbon marked with * is an asymmetric carbon, X is chloro or bromo and R.sup.2 is a branched-alkyl group. These compounds are said to exhibit strong ferroelectricity. The stereochemistry of the optically active center is not specified.
Kraus et al. in POT application EP 8,600,248, publication No. WO 8,606,373, disclose optically active nitrogen containing heterocycles which are reported to be useful as constituents for FLC materials. Phenylpyrimidines substituted with optically active groups, including haloalkyl groups are disclosed. The stereochemistry of the optically active centers is not specified.
While several useful ferroelectric liquid crystal materials (both pure compounds and mixtures) have thus bee reported, optimum response times have not been achieved (theoretical limit estimated as 10-50 nsec, Walba et al. (1986a), supra). For this reason, new FLC materials particularly those having high polarization density and low viscosity are desirable, as are new FLC dopants which can impart desired properties to FLC materials. A useful property of FLC dopants is good miscibility in smectic C* matrix materials.